Mapping chondrocyte viability, matrix glycosaminoglycan and water content on the surface of a bovine metatarsophalangeal joint
Abstract
(1) Objective
The purpose of this study was to determine if there were variations in chondrocyte viability, matrix glycosaminoglycan (GAG) and water content between different areas of the articular surface of a bovine metatarsophalangeal joint, a common and reliable source of articular cartilage for experimental study, that may compromise the validity of using multiple samples from different sites within the joint.
(2) Design
Nine fresh cadaveric bovine metatarsophalangeal joints were obtained. From each joint, sixteen osteochondral explants were taken from 4 facets, yielding a total of 144 cartilage specimens for evaluation of chondrocyte viability, matrix GAG and water content. A less invasive method for harvesting osteochondral explants and for processing the biopsy for the assessment of chondrocyte viability was developed, which maintained maximal viability within each cartilage explant.
(3) Results
There was no significant difference between the 16 biopsy sites from the different areas of the joint surface with respect to chondrocyte viability, matrix GAG and water content. Pooled data of all samples from each joint established the baseline values of chondrocyte viability to be 89.4±3.8%, 94.4±2.2% and 77.9±7.8%, in the superficial quarter, central half and deep quarter (with regard to depth from the articular surface), respectively. The matrix GAG content of bovine articular cartilage was 6.06±0.41 μg/mg cartilage, and the cartilage water content was 72.4±1.5%. There were no significant differences of these 3 variables between the different joints.
(4) Conclusions
There were no significant differences in chondrocyte viability, matrix GAG and water content in osteochondral samples taken from different areas of the joint surface. It is thus reasonable to compare biopsies obtained from different sites, as a biopsy from one site would be considered representative of the whole joint.
Key words: Joint mapping, chondrocyte viability, confocal microscopy, glycosaminoglycan content.
Introduction
Osteochondral explants are frequently used as an experimental model in cartilage research and a variety of approaches have been utilised depending on the study performed. Commonly, tissue samples take the form of full-depth osteochondral cylinders or tissue blocks with attached subchondral bone1-3. Other investigators have prepared cartilage explants with the subchondral bone carefully removed or avoided 1,4-7, or have used more specific explants such as those reflecting osteoarthritic changes8 or in which growth plate cartilage is present8,9.
In the majority of studies to date, cartilage explants have then been randomised with relatively little attention being paid to the origin of their location on the joint and therefore the results reflect the average response of the areas under investigation. However, the prevailing load has been shown to affect the cartilage thickness and matrix components of different sites within the same joint10,11. This is reflected in the variation in chondrocyte morphology, collagen fibre orientation and the type and amount of matrix proteoglycans which vary with cartilage depth 7,12-14. Thus, even though randomised cartilage explants are routinely used, it is still uncertain whether an explant from one site is representative of the whole joint.
This study was therefore designed to evaluate chondrocyte viability, glycosaminoglycan (GAG) and water content of the extracellular matrix (ECM) within carefully mapped areas of the bovine metatarsophalangeal joint surface, which is commonly used for a range of studies in the field of cartilage research4,15,16. A standard method to harvest the cartilage explant from the joint and to examine these variables was also established. The hypothesis was that there were no differences across the joint with regard to chondrocyte viability, matrix GAG and water content.
Methods
Materials
Chemicals were obtained from Sigma-Aldrich (Dorset, UK) unless otherwise stated. The cell viability probes 5-chloromethylfluorescein diacetate (CMFDA) and propidium iodide (PI) were purchased from Invitrogen (Paisley, UK) and were prepared in dimethyl sulphoxide (DMSO) as aqueous 7 μM stocks. Dulbecco’s Modified Eagle Medium (DMEM; glucose 4.5g/L) was also obtained from Invitrogen.
Harvest of the bovine osteochondral explants
Nine metatarsophalangeal joints of 3-year-old cows were obtained from a local abattoir, washed, skinned and opened under sterile conditions within 6 hrs of slaughter. Only healthy joints without macroscopic evidence of cartilage damage/degeneration were used. There were 8 facets in the joint (Figure 1A), and the 1st, 4th, 5th, and 8th facets were chosen to harvest a total of 16 osteochondral explants, as these were larger and flatter than the ridge facets (the 2nd, 3rd, 6th and 7th facets). In addition, the majority of published studies to date have harvested experimental explants from these four convex articular. Three osteochondral explants were taken from the 1st and 8th facets, namely sites A2, A3, A4 in the 1st facet and sites D2, D3, D4 in the 8th facet, and 5 osteochondral explants were obtained from the 4th and 5th facets, namely sites B1 to B5 in the 4th facet and sites C1 to C5 in the 5th facet (Figure 1B). As sharpness of the biopsy tool was crucial for taking the cartilage samples from the hard subchondral bone, a new No.22 scalpel blade was used for the acquisition of each explant. A small piece of subchondral bone was left attached in the centre of the explant to ensure that the full thickness of cartilage had been biopsied (Figure 1C). During the entire harvesting procedure, both the articular surface of the metatarsophalangeal joint and the harvested explants were kept wet by frequent rinsing with phosphate buffered saline (PBS).
Chondrocyte viability assessment
Explants were trimmed by using a rocking-motion with a custom-made double-bladed cutting tool, to create two parallel straight edges on the cartilage explant (Figure 2A-2C). The middle part of the trimmed explant was chosen and incubated (45 mins at room temperature) in DMEM with CMFDA and PI (both at a final concentration of 7 μM), labelling living chondrocytes green and dead chondrocytes red, respectively15. The approximate depth of dye penetration was 60 to 80 μm from the cut surface. Explants were subsequently fixed with 10% (v/v) formalin (Fisher Scientific, Loughborough, UK) and then stored in PBS at 4°C for 24 hrs. For confocal laser scanning microscopy (CLSM), explants were secured to the base of a Petri dish with 2 small pieces of Blu-Tack (Bostik, Leicester, UK) (Figure 2D).
An upright confocal laser scanning microscope (Zeiss LSM510 Axioskop, Carl Zeiss, Welwyn Garden City, UK) with a ×10 objective was used to acquire optical sections of CMFDA- and PI-labelled chondrocytes in the coronal plane i.e. through the cut-edge. The scanned images were reconstructed and analysed using ImageJ software (Version 1.47, NIH, USA). Articular cartilage was characterised into three regions on the basis of depth from the articular surface to the subchondral bone: the first quartile of cartilage was defined as the superficial quarter, followed by the central half as the middle 50% of the thickness, and the deep quarter as the last quartile (Figure 2E). Chondrocyte viability within each region was quantified as follows: (number of live (CMFDA-labelled) cells / total number of cells (live + dead (PI-labelled)) × 100%.
Matrix glycosaminoglycan assessment
The content of sulphated glycosaminoglycan (GAG) in the extracellular matrix of cartilage was measured using a spectrophotometric microassay method 17,18. The 1,9-dimethylmethylene blue (DMMB) solution was maintained at pH 3.0. The dilution solution was Tris/HCl (50mM) with a pH of 8.0. The standard solution was made from shark chondroitin sulphate (CS) with a concentration of 0.1mg/ml.
The biopsied cartilage explants were trimmed with a skin biopsy punch of 2.5 mm diameter (Kai Industries, Japan) to obtain the central full-thickness area of cartilage tissue. This sample was weighed to obtain its ‘before-digested’ wet weight, which included the weight of cartilage and subchondral bone. Papain (300 μg) was then added to digest the cartilage sample at 60°C for approx. 4 hrs. After digestion, 10 μl of 1M iodoacetic acid solution was added to stop the effect of papain, and the solution diluted with 4 ml Tris/HCl buffer. The undigested subchondral bone was weighed in order to calculate the true cartilage weight, which was the ‘before-digested’ wet weight of the sample subtracted from the wet weight of the subchondral bone. The absorbance of the solution was measured immediately after adding 1 ml DMMB solution (within 10 secs after mixture). The absorbance of the sample was compared with the standard solution to obtain the equivalent GAG weight of the cartilage matrix. This result (in micrograms) was then normalized to the total cartilage mass (in milligrams) to allow for any variation in the size of the cartilage specimen. Thus, GAG content was presented as the GAG mass (in μg) per cartilage mass (in mg), i.e. ‘GAG (μg/mg cartilage)’ in Results.
Cartilage water content
Excess moisture on cartilage explants was removed by placing them briefly and gently between folded filter paper prior to weighing (to obtain wet weight). The samples were then lyophilised at -55°C and 0.1 atm over 12 hrs and then weighed to obtain the dry weight with the difference in weights representing the cartilage water weight. Water content was calculated using the formula: (Cartilage water weight / wet weight of cartilage) ×100%.
Statistical analysis
Statistical analyses were performed using Minitab 16 (Minitab Inc., USA). All data were tested for normality using the Kolmogorov-Smirnov test. Thereafter, parametric data were analysed using paired or unpaired Student’s t-tests if two sets of data were compared, or one-way ANOVA with post hoc Tukey’s tests if more than two sets of data were analysed. For non-parametric data, the Mann-Whitney U test was used for comparison of two sets of independent results, while the Kruskal-Wallis test was used for three or more sets of non-parametric data. Data are presented as means ± standard deviation (SD) with the level of significance set at p < 0.05.
Results
A total of 9 different joints were used to test the 3 variables (3 joints for each variable) which were (a) chondrocyte viability, (b) GAG content and (c) water content. For chondrocyte viability, the results from a total of 48 sites (3 joints, 16 sites per joint) showed that viability in the superficial quarter, central half and deep quarter was 89.4±3.8%, 94.4±2.2% and 77.9±7.8%, respectively (Table 1). Statistical analysis revealed that there were no significant difference between the three regions (p=0.620, 0.787, and 0.361 in the superficial quarter, central half and deep quarter, respectively, one-way ANOVA).
The matrix GAG content, measured in 48 sites of 3 different joints, was 6.06±0.41 μg/mg cartilage (Table 2). The one-way ANOVA indicated that there was no statistically significant difference of the matrix GAG content between each site (p=0.165). Similarly, the water content of different sites of the articular surface was not significantly different (p=0.628, one-way ANOVA, with an average content of 72.4±1.5%) (Table 3).
Further comparisons between joints were performed demonstrating no significant difference between individual metatarsophalangeal joints of bovines in terms of their chondrocyte viability, matrix GAG or water content (Table 4). In summary, these results suggest that a full-depth osteochondral sample taken from any one of the sites described in this bovine joint would be representative of the cartilage throughout the joint.
Discussion
The results from these 48 biopsy sites supported the hypothesis that there was no difference between the sites with respect to chondrocyte viability, matrix GAG or water content. The findings indicated that for these characteristics, a cartilage sample from any of these sites on the joint surface was representative of the whole joint. The results also confirmed that there was no statistical difference between these variables in joints from different individuals of the same species. However, the variability of the data was least for sites B3 and C3, both of which were located in the middle of the articular surface, which may indicate that these sites could be more suitable than others if only one or two cartilage samples are required from each bovine joint.
Characterisation of chondrocyte viability, matrix proteoglycan and water content in fresh (Day 0) joints was important because the data would provide baseline values for comparison with the results obtained under different experimental conditions. The data demonstrated that there would not be a sampling bias when the specimens were obtained from different locations of the joint surface. A knowledge of the variability of these values is also useful in the experimental design phase for power calculations. We are not aware of any data on this in the current literature even though cartilage explants from bovine joints have been used extensively for many years. The data from this joint mapping study helps to rectify this deficiency.
From these results, it could be established that the baseline values of chondrocyte viability were 89.4±3.8%, 94.4±2.2% and 77.9±7.8%, in the superficial quarter, central half and deep quarter, respectively. The matrix GAG content of bovine articular cartilage was 6.06±0.41 μg/mg cartilage, and the cartilage water content was 72.4±1.5%. However, when comparing the results from different studies, it is important to take into account the differences in the materials and methods between the studies. For example when studying human surgical specimens, Amin et al. (2008) reported that chondrocyte viabilities within cartilage explants from human knee joints were 86.4%, 91.9% and 82.2% in the superficial quarter, central half and deep quarter, respectively19. Pun et al. (2006) also studied cartilage explants of human knee joints and demonstrated that chondrocyte viability on day 0 was 80.5%, 80.0% and 83.0% for the superficial quarter, central half and deep quarter, respectively20. Chondrocyte viability in these surgical specimens may have been reduced because of (i) cartilage degeneration itself or, (ii) as a result of the surgical manipulation of the cartilage explant or (iii) due to the cutting action – as uncut cartilage would be expected to show less chondrocyte death.
The measured water content of cartilage in the current study was 72.4±1.5%. This was the average value across all the areas in the present study, and is in agreement with the randomised samples that have been used in previous reports21. For the GAG content of the extracellular matrix, Hoemann et al. (2002) reported that fresh cartilage explants from the bovine shoulder joint contained 4.9 to 5.8 μg/mg cartilage22. Their values were slightly lower than the results presented here, which may have been due to the samples originating from a different joint with a different loading pattern, however it was perhaps more likely due to the different harvesting technique. The explants in their study were harvested from the joint directly with 6 mm biopsy punches. Due to the hardness of the subchondral bone, this biopsy technique may have caused more stress to the cartilage explants than the technique used in the current study. This might have resulted in more matrix GAG loss from the cutting margin of the biopsied explants.